This invention is related generally to an apparatus and method for measuring stratospheric ozone. More specifically, this invention is related to an instrument that employs a high-resolution fiber-optic spectrometer, coupled with precisely aimed fiber optics to acquire intensity data at multiple UV wavelengths within the UV-B range (280-320 nm). In this wavelength range, UV is attenuated by ozone, but not all is dissipated, as is the case for UV-C (100-280 mm). Although not wanting to be bound by theory, the equation used to calculate ozone incorporates the ratio of intensity for multiple wavelengths in order to eliminate interference effects from aerosols.
Life processes on earth are driven by energy which originates from the sun or has been stored from past solar radiation. The sun provides a broad spectrum of light energy, most of which is beneficial, and some of which is harmful, dependent upon exposure level. Exposure to full spectrum solar UV light, as it exists high in our atmosphere, is damaging to both biotic and abiotic entities. We therefore rely on most of this UV energy being filtered out before reaching the earth's surface. This filtering is done effectively by the earth's middle atmosphere, primarily by ozone in the stratosphere. The invention described herein is concerned with an cost effective apparatus for measuring stratospheric ozone for purposes of monitoring the Earth's UV filter.
The discovery of stratospheric ozone depletion over Antarctica provided impetus to the ozone depletion theory of Molina and Rowland and prompted the creation of networks to monitor changes in stratospheric ozone and related enhanced levels of ultraviolet radiation over the surface of the Earth, especially the biologically active UV-B. Estimates suggest that UV irradiance has increased since the early 1980s over mid- and high latitudes of both hemispheres.
Increased levels of UV have potential consequences for human health, agricultural systems, ecological systems, and material goods. Some human effects are DNA damage in cells which can lead to skin cancer, premature aging of the skin, increased risk of cataract development, and immune system repression.
Excessive UV-B has been shown to affect the physiological and developmental processes of plants. This has obvious agricultural implications for UV intolerant plant species and may necessitate the development of tolerant hybrids. The potential effects of increased levels of UV-B exposure on crops and ecosystems are not fully understood, but experiments have shown that increased UV-B exposure, especially during plant development, may induce effects such as reduction in grain yield and susceptibility to disease. Examples are reduced leaf area and DNA damage due to exposure during plant development that is not fully repaired and thus inherited by offspring and accumulated over generations, and reductions in dry matter accumulation that affect maize grain yield. Further effects of UV-B may be felt in forests and grasslands with implications on succession and biodiversity.
Recent decreases in concentration and emission rates of ozone-depleting compounds from their peak in the early 1990s indicate potential ozone recovery to pre-1980 values by the middle of the century. Observations during the 1990s indicate that ozone depletion is also occurring over the Arctic but the formation of an ozone hole similar to the one over Antarctica is unlikely. Ozone at middle latitudes has also been reduced. The average total column ozone (1997-2001) was 3% and 6% below the pre-1980 values in the Northern and the Southern Hemisphere respectively. Clearly, contributions from networks of ground-based instruments and satellite observations have greatly contributed to this understanding of ozone dynamics. Monitoring of ozone is useful while recovery of ozone occurs, due to global efforts to curb the use of products releasing chemicals harmful to the ozone layer.
Improving the spatial distribution of observing ground stations, for continued and expanded global monitoring of UV and ozone, is needed to assess current and future trends and to support satellite measurements. However, existing automatic total column ozone monitors are complex and expensive instruments. The instrument developed in this dissertation is a low-cost, yet automated, total column ozone instrument that permits the widespread distribution of monitors with potential to greatly improve our understanding of ozone dynamics.
Current Stratospheric Ozone and UV Monitoring Programs in US include the USEPA UV-Net consists of a network of 21 Brewer spectrophotometers in the United States that measure solar radiation in the LV-B and UV-A bands. Irradiance data are analyzed and disseminated to scientists and the public to assess distribution and trends of UV and long-term records for impact studies on biota and materials. Fourteen of these UV monitoring sites are located in National Parks and are operated by the National Park Service as part of the Park Research and Intensive Monitoring of Ecosystems Network (“PRIMENet”). The other seven sites correspond to urban areas. The National Ultraviolet Monitoring Center (“NUVMC”) of the University of Georgia helps operate the UV-Net that is coordinated with programs of other U.S. Federal agencies through the U.S. Global Change Research Program. Total column ozone can be derived from the Brewer spectral measurements. One of ordinary skill in the art realizes that most US programs related to UV and ozone can be accessed via the UV-Net web page.
The UV Index forecast of the National Oceanic and Atmospheric Administration (“NOAA”) Climate Prediction Center is a joint effort with US-EPA to produce a daily forecast of UV Index for 58 US cities as a text bulletin and in map form. It provides historical data in the form of past bulletins, monthly means and maxima and annual time series plots. NOAA's Aeronomy Laboratory (“AL”), Climate Monitoring and Diagnostics Laboratory (“CMDL”), Climate Prediction Center (“CPC”) and the National Climatic Data Center (“NCDC”) are involved in monitoring and research of ozone and the processes affecting its concentration in the stratosphere. CMDL maintains 15 stations based on the Dobson Ozone spectrophotometer, and is the World Dobson Ozone Calibration Center of Global Atmosphere Watch (“GAW”). This center is responsible for the calibration of about 100 instruments worldwide. Most of the ground-based total ozone instruments are part of the GAW program coordinated by the World Meteorological Organization (“WMO”). NOAA's Surface Radiation Research Branch operates the Central UV Calibration Facility (“CUCF”), the U.S. SurfRad Network of solar radiation instruments and conducts research on Uw Radiation and its effects on the earth.
In response to predictions of increased UV radiation in the polar regions, the National Science Foundation established, in 1988, the UV Monitoring Network, which currently consists of six high resolution spectroradiometers (e.g. BSI model SUV-100) located in Antarctica and other high latitude regions with a reference site in San Diego, Calif. The network perfoms measurements of global spectral irradiance in the UV and visible bands. Data are provided to researchers studying the effects of ozone depletion on terrestrial and marine biological systems.
The US Department of Agriculture (“USDA”) maintains a UV-B Monitoring and Research Program initiated in 1992 to provide information on the geographical distribution and temporal trends of UV-B in the United States. This information is useful to the assessment of the potential impacts of increasing UV-B levels on crops and forests. The research network provides high resolution spectroradiometers to six selected sites.
Available Ozone Detection Instruments. The oldest ozone spectrophotometer was designed by Dobson in the 1920's, and uses prisms to select from two to six different wavelengths (between 305 and 345 nm) of the incoming light; it is still considered as the reference instrument for total ozone observations. Dobson instruments are regularly calibrated at international comparisons performed under the GAW program of WMO and related to the primary standard maintained by NOAA CMDL, Boulder, Colo.
The Brewer measures direct and global (direct+diffuse) spectral irradiance at high resolution in the wavelength range of 290-372 nm. For example, the MKIV provides spectral irradiance data in the range of 286.5 to 363 nm in steps of 0.5 nm. Total ozone is derived from direct sun measurements at four UV wavelengths. The Brewer allows automated measurements. For cloudy sky conditions, ozone may be derived from scattered UV radiation from the zenith direction. Concentration of SO2 can also be derived from the instrument measurements.
The SUV-100 (Biospherical Instruments) is also high resolution (nominal bandwidth 1 nm) and is used by NSF Polar Region UV monitoring network. Based on a double monochromator and gratings, the SUV-100 is driven by a stepping motor with a step size of 0.05 nm. Other moderate resolution instruments are also used. For example the GUV 511 (Biospherical Instruments) measures UV irradiances in four channels, with center wavelengths at 305 nm, 320 nm, 340 nm and 380 nm. Bandwidths are approximately 10 m Full Width at Half Maximum (“FWHM”).
The Microtops II hand-held ozonometer, manufactured by Solar Light Company, is a manual instrument which measures total column ozone and water vapor. The Microtops II is a five-channel sun-photometer with center wavelengths at 305.5, 312.5, 320, 940, and 1020 nm. The UV channels (305.5, 312.5, 320 nm) have a FWHM resolution of 2.5 nm and are used for total ozone column calculations.
Ozone Measurements from Satellites can be determined using a Total Ozone Mapping Spectrometer (“TOMS”) provides data on UV and ozone. TOMS provides global distribution of ozone as well as total ozone estimates over selected sites. Backscattered radiation levels at wavelengths where ozone absorption does and does not take place are compared with the same wavelengths measured directly from the sun to derive a total ozone amount in the earth's atmosphere. This methodology is also used by NOAA's Solar Backscatter Ultraviolet (“SBUV/2”) instrument. Ozone cannot be provided for the earth's shadow or polar night regions because ozone estimates are based on UV measurements. The SBUV/2 instrument currently onboard the NOAA-16 polar orbiting satellite measures the ultraviolet sunlight scattered by the Earth's atmosphere at several wavelengths ranging from 252 to 340 nm. Measurements at the shortest eight wavelengths are used to estimate ozone vertical profiles. Measurements at the longer four wavelengths, which penetrate into the lower atmosphere, are used to obtain estimates of total column ozone. It uses the ratio of two wavelengths of backscattered ultraviolet light where one is strongly absorbed by ozone while the other is absorbed very little. NOAA's TIROS Operational Vertical Sounder (“TOVS”) instruments measure infrared radiation emitted by the Earth at several wavelengths instead of ultraviolet backscattered radiation. Thus, the TOVS can determine ozone at night.
Because ozone in the stratosphere is beneficial, it is often referred to as “good” ozone, compared to ozone at ground level which is frequently called “bad” ozone. Ozone in the stratosphere protects us from harmful UV, whereas ground level ozone causes respiratory problems in humans and can be deadly to plants. The apparatus and method described herein concerns the measurement of “good” ozone. Therefore, unless specified otherwise, the word ozone implies “good” ozone throughout this specification.
Ozone is a triple oxygen molecule that forms when a free oxygen atom joins with an O2 molecule to form O3. Both “good” and “bad” ozone have the same chemical structure, the only difference being the location in the atmosphere. The first step in ozone formation is the generation of a free oxygen atom. At ground level, the oxygen atom is produced when UV light bombards the air pollutant nitrogen dioxide, freeing one of the oxygen atoms according to the following reaction:NO2+UV→NO+OIn the stratosphere free oxygen is obtained primarily by the following reaction:O2+UV→O+Owhere the UV photon energy required to split an O2 molecule is much higher than that needed to separate the oxygen atom from NO2. In order to split an O2 molecule, the UV wavelength should be less than or equal 240 nm. Recall Planck's lawE=hνwhere E is energy, h is Planck's constant and ν is frequency, which in turn is written as the quotient of the speed of light (c) over wavelength (λ)ν=c/λ.Thus, the shorter the wavelength, the higher the energy. Only photons with λ≦240 nm have enough energy to break O2 bonds. Once a free oxygen atom is available, O3 is formed by the reactionO2+O→O3.
In the stratosphere, the process of generating atomic oxygen by splitting O2 molecules is slow due to the limited solar energy below 240 nm and, as a result, the creation of new ozone from oxygen is a slow process.
UV light is categorized as UV-A, UV-B, and UV-C dependent upon wavelength. Generally, light with a wavelength in the range 100-280 nm is considered UV-C, 280-320 nm is UV-B, and 320-400 nm is UV-A. By this definition, only UV-C has enough energy to split an O2 molecule, but the total amount of UV-C available is much lower than UV-B or UV-A, resulting in a slow process for generating stratospheric ozone. The ozone layer is effective at filtering out UV-C and substantially none of it reaches the troposphere. UV-B is partially filtered by ozone whereas very little UV-A is filtered. It is therefore the biologically active UV-B with which we are primarily concerned as ozone thickness varies for any reason.
The process by which ozone absorbs and thereby attenuates harmful UV before it reaches the lower atmosphere is destruction of the ozone molecule itself. When UV light strikes an O3 molecule it breaks apart into an O2 molecule and an oxygen atom and the UV photon is absorbed. However, the free oxygen, usually very quickly, reacts with another O2 molecule to reform O3. This process of breaking and reforming O3 is a fast process due to the fact that the photon energy required to break apart O3 is not as high as for O2 and due to the relative abundance of O2. O3 can be split by a photon with λ≦325 nm. There is an abundance of UV light in the stratosphere at these wavelengths, including some UV-A and the entire range of UV-B, and UV-C. FIG. 1 illustrates insight into the ozone creation and destruction process. Without unnatural trace gases present in the stratosphere, ozone concentration stays in balance as it is quickly destroyed by UV and then reformed. As shown in FIG. 1, some oxygen atoms escape the quick cycle by forming O2 and returning to the slow process at the left of the figure (path 3), but other oxygen atoms are produced by high energy UV and these atoms reenter the fast ozone process to the right (path 1).
This cycle of ozone destruction and creation continues in a relative equilibrium fashion unless trace gases such as chlorine or bromine become more abundant and react with the free oxygen atoms, removing them from the cycle. Both chlorine and bromine found in the stratosphere are derived from man made chemicals that are transported to the stratosphere, break down, and destroy many ozone molecules.
The primary source of chlorine in the stratosphere is from the breakdown of chlorinated fluorocarbons (“CFCs”) used as refrigerants, and formerly as propellants and for cleaning manufactured subassemblies. While CFCs are very stable in the troposphere and are somewhat heavier than air, they are nevertheless carried to the stratosphere by thermally driven currents called Brewer-Dobson circulation. Once CFCs reach the stratosphere, ultra-high energy UV breaks them down. This ultra-high energy UV is not present in the troposphere and CFCs are thus very stable at ground level.
Although not wanting to be bound by theory, the following reaction equations show how CFCs serve as a catalyst for ozone destruction. First, chlorine is freed from a CFC molecule (Freon-11 in this case) by a high energy UV photon and then it reacts with O3 to produce chlorine monoxide. The chlorine monoxide then further consumes a free oxygen atom which would have potentially formed ozone. The net effect is that two oxygen molecules are produced from one ozone molecule and one free oxygen atom, an effective loss of two ozone molecules.
                                                                        CFCl                3                            +              UV                        ->                        ⁢                                          CFCl                2                            +              Cl                                                                                      Cl              +                              O                3                                      ->                        ⁢                          ClO              +                              O                2                                                                                                    ClO              +              O                        ->                        ⁢                          Cl              +                              O                2                                                          _                      ⁢                            NET          ⁢                      :                    ⁢                                          ⁢                      O            3                          +        O            ->              2        ⁢                                  ⁢                  O          2                    
At the end of the above set of reactions, the chlorine is still free to destroy more ozone and the cycle goes on until the chlorine atom finds another atom such as hydrogen, reacting to form HCl. At that point the damage finally ceases. It is estimated that one free chlorine atom destroys about 1000 ozone molecules before being taken out of circulation.
The apparatus and method of this invention was developed to be a substantially automated low-cost instrument to measure stratospheric ozone. Although not wanting to be bound by theory, the availability of this apparatus and method enables broader ground-based observations from repeatable positions, contributing to an expansion of UV and ozone data, and enabling a more complete mapping of ozone density. Additionally, data recorded by such instruments will support satellite observations and provide ozone data on days when satellite coverage is not available, due to satellite instrument field of view and longitudinal location of its orbit. This apparatus and method have a flexible platform, which may be adapted to study other atmospheric constituents with potential applications in a variety of ecosystem and air quality studies.
The new instrument employs a high-resolution fiber-optic spectrometer, coupled with precisely aimed fiber optics to acquire intensity data at multiple UV wavelengths within the UV-B range (280-320 nm). In this wavelength range, UV is attenuated by ozone, but not all is dissipated, as is the case for UV-C (100-280 mm). The equation used to calculate ozone incorporates the ratio of intensity for multiple wavelengths in order to substantially eliminate interference effects from aerosols. Wavelength selection was guided by past efforts such as the Dobson, Brewer, and MICROTOPS II instruments. Automation allows continuous monitoring and logging with only occasional periodic human intervention.
The invention described herein comprises:    1) A system architecture with specific system components;    2) Software algorithms used to collect and process UV-B spectral data;    3) A sun tracker based upon a two axis positioning mechanism;    4) An appropriate control method capable of integrating the entire system and developed software into a substantially automatic system;    5) Instrument calibration procedures including automated collection of Langley data;    6) The ability to calculate total column ozone using UV-B data from the spectrometer and save the intensity data and result in a file for future analysis; and    7) The ability to characterize the entire light collection sub-system including fiber optics, lenses, filters and pan and tilt mechanism;
The precision and accuracy of the instrument was determined by comparing its performance against MICROTOPS II and TOMS satellite data. The instrument's performance was evaluated to generate conclusions about the recommend methods of use for this apparatus. One of ordinary skill in the art will understand that ground based units are less expensive than satellites.
Additionally, ground based, conventional stratospheric ozone measuring instruments cost about $200,000 in 2005 U.S. dollars. The cost of these instruments greatly restricts the number that can be deployed for measuring stratospheric ozone. In contrast, the instrument, system and method of the present invention comprise a total column ozone measuring instrument which, advantageously, can be built for substantially less than a conventional instrument. The low cost of the present invention will permit expanded global monitoring of stratospheric ozone. Furthermore, the present invention is adapted to measure other trace gases and haze in the atmosphere.